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Análise teórico-prática do risco de cheias no Arquipélago da Madeira: o caso de estudo dos Concelhos do Funchal, Machico, Ribeira Brava e São Vicente
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Authors
Abstract(s)
Com base no histórico recente de cheias na Ilha da Madeira, evidencia-se que fenómenos anteriormente
considerados raros tornaram-se cada vez mais frequentes, impulsionados pelas mudanças climáticas
globais. Além do aspeto cíclico da variação climática atuante no planeta, as ações antrópicas
contribuíram consideravelmente para o agravamento desta problemática, principalmente no que se
refere às cheias. Este fenómeno ocorre pelo crescimento desordenado das cidades que tem ocasionado
um aumento do índice de impermeabilização do solo, sendo este, o principal responsável pelo
escoamento superficial e as consequentes inundações dos centros urbanos.
Os cursos de água de uma bacia hidrográfica são os responsáveis por canalizar todo volume de água
precipitado para um ponto em comum: a foz. Neste sentido, torna-se primordial a verificação das
características intrínsecas destes canais – i.e. tipo de canal (natural ou artificial), coeficiente de
rugosidade, altura, largura, declividade e principalmente a capacidade de escoamento. Uma vez que todo
o escoamento superficial será direcionado para a foz, é essencial que a sua capacidade de escoamento
seja maior que o caudal presente, pois de acordo com a Lei da Continuidade Hidráulica conjugada à
Equação de Manning-Strickler, com a redução da velocidade da água (principalmente pelo coeficiente
de rugosidade do leito) a manutenção do caudal dependerá do aumento do nível da água, possibilitando
o transbordo da ribeira e todos os transtornos supracitados.
Referente à Ilha da Madeira, os cursos de água principais possuem trechos naturais e artificiais. As
partes artificiais fazem-se pela necessidade de aumentar a secção de escoamento e a redução da perda
de velocidade por atrito com o leito, utilizando o betão como revestimento. Porém, nota-se que
atualmente estas ribeiras possuem demasiada sedimentação e também a presença de vegetação,
retornando às características parciais de um canal natural. Para uma análise mais precisa da capacidade
de vazão destes trechos é imprescindível considerar estas problemáticas, já que a deposição de
sedimentos além de reduzir a velocidade da água, ocasionará a redução da secção de escoamento.
Com base em uma extensa revisão bibliográfica, foram reunidas diversas metodologias e parâmetros
necessários para a caracterização das bacias hidrográficas mais problemáticas com maior precisão. Além
disso, verificou-se as diretrizes estabelecidas pelas instituições regionais no que se refere ao
Planeamento Urbano, como forma de efetuar um estudo que siga a legislação vigente.
Portanto, o presente estudo pretende efetuar uma análise comparativa entre os diversos parâmetros
indicativos de suscetibilidade às cheias – i.e. densidade de drenagem; tempo de concentração;
dimensões; fator de forma; índice de compacidade e etc. – das bacias hidrográficas de quatro concelhos
da Ilha da Madeira, nomeadamente: Funchal, Machico, Ribeira Brava e São Vicente. Após a
determinação dos parâmetros supracitados, proceder-se-á também à análise da capacidade de
escoamento a jusante, de forma a verificar a propensão ao transbordo de acordo com o caudal de ponta
de cheia expectável para as respetivas bacias hidrográficas. Para este estudo será utilizado o software
ArcGIS, elaborado pela ESRI, o qual permite obter valores mais precisos e confiáveis em comparação
com a metodologia cartográfica clássica. Verificada a incapacidade de escoar o caudal precipitado
estimado para um tempo de retorno de 100 anos, será dimensionada uma Bacia de Detenção para a
regularização e controlo do escoamento, de forma a permitir que a foz funcione dentro de sua
capacidade.
Por fim, pretende-se também abordar a importância de um planeamento urbano que vise reduzir os
impactos ambientais e mitigar os efeitos destrutivos dos fenómenos climáticos extremos, de forma a
diminuir as perdas materiais, humanas e sociais na região.
Based on the recent history of flooding in Madeira, it is evident that phenomena previously considered rare have become increasingly frequent, driven by global climate change. In addition to the cyclical aspect of climatic variation operating on the planet, anthropic actions have contributed considerably to the aggravation of this problem, especially regarding to floods. This phenomenon occurs due to the disordered growth of cities that has caused an increase in the impermeabilization index of soil, which is the main responsible for surface runoff and the consequent flooding of urban centers. The water courses of a hydrographic basin are responsible for channeling all the precipitated water volume to a common point: the stream mouth. In this sense, it is essential to check the intrinsic characteristics of these channels - i.e. type of channel (natural or artificial), roughness coefficient, height, width, slope and mainly the flow capacity. Since all the surface flow will be directed to the stream mouth, it is essential that its flow capacity is greater than the present flow, because according to the Law of Hydraulic Continuity combined with the Manning-Strickler Equation, with the reduction of speed of the water (mainly due to the channel’s roughness coefficient) the maintenance of the flow will depend on the increase of the water level, allowing the overflow of the stream and all the aforementioned disorders. Regarding to Madeira Island, the main water courses have natural and artificial stretches. The artificial parts are made by the need to increase the flow section and reduce the loss of speed by friction with the channel, using concrete as a coating. However, it is noted that currently these streams have too much sedimentation and also the presence of vegetation, returning to the partial characteristics of a natural channel. For a more accurate analysis of the flow capacity of these stretches it is essential to consider these elements, since the deposition of sediments, in addition to reducing the speed of the water, will cause the reduction of the flow section. Based on an extensive bibliographic review, several methodologies and parameters necessary to characterize the most problematic hydrographic basins with greater precision were gathered. In addition, the guidelines established by the regional institutions with regard to Urban Planning were verified, as a way of carrying out a study that follows the current legislation. Therefore, the present study intends to carry out a comparative analysis between the various parameters indicative of susceptibility to floods - i.e. drainage density; concentration time; dimensions; form factor; compactness index and etc. - the hydrographic basins of four cities on the Madeira island, namely: Funchal, Machico, Ribeira Brava and São Vicente. After determining the aforementioned parameters, an analysis of the downstream flow capacity will also be carried out, in order to verify the propensity to overflow according to the expected peak flow rate for the respective hydrographic basins. For this study, the ArcGIS software, developed by ESRI, will be used, which allows to obtain more accurate and reliable values in comparison with the classic cartographic methodology. Having verified the inability to drain the estimated precipitous flow for a return time of 100 years, a Detention Basin will be designed to regulate and control the flow, in order to allow the stream mouth to function within its capacity. Finally, it is also intended to address the importance of urban planning that aimsto reduce environmental impacts and mitigate the destructive effects of extreme climatic phenomena, in order to reduce material, human and social losses in the region.
Based on the recent history of flooding in Madeira, it is evident that phenomena previously considered rare have become increasingly frequent, driven by global climate change. In addition to the cyclical aspect of climatic variation operating on the planet, anthropic actions have contributed considerably to the aggravation of this problem, especially regarding to floods. This phenomenon occurs due to the disordered growth of cities that has caused an increase in the impermeabilization index of soil, which is the main responsible for surface runoff and the consequent flooding of urban centers. The water courses of a hydrographic basin are responsible for channeling all the precipitated water volume to a common point: the stream mouth. In this sense, it is essential to check the intrinsic characteristics of these channels - i.e. type of channel (natural or artificial), roughness coefficient, height, width, slope and mainly the flow capacity. Since all the surface flow will be directed to the stream mouth, it is essential that its flow capacity is greater than the present flow, because according to the Law of Hydraulic Continuity combined with the Manning-Strickler Equation, with the reduction of speed of the water (mainly due to the channel’s roughness coefficient) the maintenance of the flow will depend on the increase of the water level, allowing the overflow of the stream and all the aforementioned disorders. Regarding to Madeira Island, the main water courses have natural and artificial stretches. The artificial parts are made by the need to increase the flow section and reduce the loss of speed by friction with the channel, using concrete as a coating. However, it is noted that currently these streams have too much sedimentation and also the presence of vegetation, returning to the partial characteristics of a natural channel. For a more accurate analysis of the flow capacity of these stretches it is essential to consider these elements, since the deposition of sediments, in addition to reducing the speed of the water, will cause the reduction of the flow section. Based on an extensive bibliographic review, several methodologies and parameters necessary to characterize the most problematic hydrographic basins with greater precision were gathered. In addition, the guidelines established by the regional institutions with regard to Urban Planning were verified, as a way of carrying out a study that follows the current legislation. Therefore, the present study intends to carry out a comparative analysis between the various parameters indicative of susceptibility to floods - i.e. drainage density; concentration time; dimensions; form factor; compactness index and etc. - the hydrographic basins of four cities on the Madeira island, namely: Funchal, Machico, Ribeira Brava and São Vicente. After determining the aforementioned parameters, an analysis of the downstream flow capacity will also be carried out, in order to verify the propensity to overflow according to the expected peak flow rate for the respective hydrographic basins. For this study, the ArcGIS software, developed by ESRI, will be used, which allows to obtain more accurate and reliable values in comparison with the classic cartographic methodology. Having verified the inability to drain the estimated precipitous flow for a return time of 100 years, a Detention Basin will be designed to regulate and control the flow, in order to allow the stream mouth to function within its capacity. Finally, it is also intended to address the importance of urban planning that aimsto reduce environmental impacts and mitigate the destructive effects of extreme climatic phenomena, in order to reduce material, human and social losses in the region.
Description
Keywords
Escoamento superficial Gestão ambiental Hidrologia Inundações Palneamento urbano Surface runoff Environmental management Hydrology Floods Urban planning Engenharia Civil . Faculdade de Ciências Exatas e da Engenharia